1. Field of the Invention
The present invention relates generally to a method of controlling a double clutch transmission (DCT) of a vehicle and, more particularly, to a technique for preventing shift shock from occurring when shifting gears in response to deceleration of a vehicle provided with a DCT.
2. Description of Related Art
A downshifting process of a DCT when a vehicle provided with the DCT decelerates, for example, to stop, will be described below with reference to FIG. 1.
FIG. 1 illustrates a process of downshifting from the preceding gear N+1 to a subsequent gear N−1 via a current gear N when decelerating a DCT vehicle including a first input shaft and a second input shaft which are respectively coupled to a first clutch and a second clutch. When in the current gear N, the first clutch which has been in an engaged state in the preceding gear N+1 is released, and the second clutch engages to allow transmission of torque. Thereafter, contrary to this, when in the subsequent gear N−1, the first clutch enters the engaged state to allow transmission of torque, and the second clutch is released.
To shift from the current gear N to the subsequent gear N−1, synchronization of a synchronizer takes place to engage a shift gear of the subsequent gear N−1, following engaging the first clutch and releasing the second clutch so that power is transmitted through the shift gear of the subsequent gear N−1.
However, during the synchronization conducted to engage a shift gear of the subsequent gear N−1, shift shock may occur. In detail, if the rpm of the first input shaft is less than that of the output shaft when engaging the shift gear of the subsequent gear N−1, backlash of elements pertaining to transmission of power between the input shaft and the output shaft, which has been arranged in one direction when synchronization takes place to engage the shift gear of the subsequent gear N−1, is rearranged in the reverse direction when the synchronization is completed or the transmission of power from the input shaft to the output shaft begins. At this time, shift shock and noise occur.
In other words, during synchronization for engagement of the shift gear of the subsequent gear N−1, if the rpm of the first input shaft coupled to the first clutch is less than that of the output shaft, when synchronization begins to take place as the sleeve moves, torque is applied from the output shaft in a direction in which the first input shaft is operated. At this time, backlash of elements, for example, a hub, the sleeve, shift gears, etc., which pertain to a power transmission path from the output shaft to the first input shaft is arranged in one direction.
Subsequently, when the sleeve further moves, the sleeve passes over a synchronizer ring and a key. Thereby, the compression force with which the sleeve pushed the synchronizer ring is removed, resulting in a momentary breakdown of the synchronization.
During breakdown of the synchronization, if there is no frictional resistance induced as a result of the rotation of the first input shaft, the first input shaft may rotate freely and thus maintains the rpm when in the synchronized state. The rpm of the output shaft reduces because the vehicle is decelerating. As a result, just before the sleeve which has further moved engages with the clutch gear, the rpm of the first input shaft exceeds that of the output shaft.
In this state, if the sleeve just moves and engages with the clutch gear, torque is applied to the elements in the direction in which the first input shaft that is faster than the output shaft rotates the output shaft. Thus, the backlash of the elements that have been arranged in one direction when the synchronization begins are rearranged in the reverse direction. Because of such a phenomenon, the corresponding elements between the first input shaft and the output shaft that have been rotating at high speeds collide with each other, thus causing shock and noise.
Such shift shock and noise which are induced during the above-mentioned process of the conventional technique deteriorate the quietness of the vehicle, causes a user discomfort, and reduce the marketability of the vehicle.
For reference, the circle portion A of FIG. 1 means shock and noise which occur from the first input shaft when shifting from the current gear N to the subsequent gear N−1, and the circle portion B indicates the shock and noise which are caused by the second input shaft when shifting from the subsequent gear N−1 to a lower subsequent gear.
The information disclosed in this Background of the Invention section is only for enhancement of understanding of the general background of the invention and should not be taken as an acknowledgement or any form of suggestion that this information forms the prior art already known to a person skilled in the art.